The present invention relates to an electric fuel pump device for use in pumping a liquid fuel from a fuel reservoir to a fuel consuming equipment. More particuarly, the invention is concerned with an electric fuel pump device for pumping a liquid fuel from a fuel tank to the combustion chamber of an engine of a vehicle.
In general, electronic fuel injecting pump system of vehicle engine incorporates an electric fuel pump which is adapted to pump a liquid fuel from the fuel tank to the combustion chamber of the engine, at a comparatively high pressure of 2 to 3 Kg/cm.sup.2. In order to obtain the comparatively high pressure, constant volume type pump is used in the electric fuel pump used for this purpose.
Although some fuel pump devices incorporate centrifugal pumps, the use of such fuel pump devices is limited only to the cases where the discharge pressure is as low as or less than 1 Kg/cm.sup.2. The fuel pump apparatus employing a constant volume type pump cannot acquire the desired performance unless it is fabricated at a high precision, so that the production cost is raised and, in addition, the levels of the vibration and noise are inconviniently increased due to a large pulsation of the discharge pressure. To the contrary, the fuel pump device incorporating a centrifugal pump is hardly operative to provide a high discharge pressure at small flow rate, although it is suitable for providing a large flow rate at a comparatively low pressure.
In order to obviate the above-described shortcomings of the prior art, the present inventors have proposed a fuel pump device incorporating a regenerative pump in the pumping section thereof. The regenerative pump, which is referred to also as "Wesco pump" can provide a high discharge pressure without any pulsation at a reduced level of noise. It is possible to easily obtain a high pressure of 2 to 3 Kg/cm.sup.2 by using a regenerative pump, particuarly a regenerative pump having an impeller of closed vane type. In the use of the regenerative pump of this type, however, it is necessary to keep suitable distances or clearances between both axial end surfaces of the impeller and the axial inner surfaces of the pump casing, for otherwise the axial end surface may inconveniently contact the opposing axial inner surface of the casing to generate a friction which in turn increases the driving torque to seriously deteriorate the pump performance. The following two ways can be taken as countermeasures for overcoming these problems. The first way is to precisely locate and fix the impeller on the rotor shaft, while the second way is to maintain a balance of pressure between both sides of the impeller while mounting the latter axially movable. These countermeasures, however, require the parts have to be fabricated at a considerably high precision, resulting in a raised cost of production.